This invention relates to vapor pressure detection and diagnostics. Most specifically, the invention relates to detection of gasoline vapor pressure and its application to fuel control in an internal combustion engine.
There are instances where one wants to know the vapor pressure of a volatile liquid for control and/or diagnostic purposes, but direct and/or immediate vapor pressure measurement is not practical by ordinary techniques. In addition, one might want to know the vapor pressure of the liquid as it actually exists under the conditions of its storage container. In one example, one may simply want to quickly identify the type of fuel contained in an automobile fuel tank or in a bulk storage tank. Another example involves automotive internal combustion engine control systems. Precise and immediate knowledge of fuel vapor pressure, i.e., fuel volatility, is often desired in automotive engine control systems. For fuel control of an automobile internal combustion engine, fuel volatility is generally measured as reid vapor pressure (RVP), which is vapor pressure measured at 100xc2x0 F. under a specified manner of measurement. High gasoline RVP improves engine startability and driveability at low ambient temperatures but can have a negative effect on precision of certain fueling system diagnostics. It is recognized that vapor pressure, and more specifically RVP, can vary significantly among available fuels. Hence, it is desirable that the fuel control system be able to identify the type of fuel being used and, more specifically, vapor pressure of the fuel being used. As pointed out in my earlier U.S. Pat. No. 5,884,610 Reddy, it is desirable to determine the precise vapor pressure, usually as RVP, of the fuel being used by an automobile""s internal combustion engine. It is desired so that the fueling system control and diagnostics can be adjusted to match it. This provides increased emission control, driveability and integrity of diagnostics.
However, just knowing the RVP of the fuel being furnished to an engine is not enough information to adequately control the ratio of air and fuel being supplied to the engine if the engine has an EVAP system. Most United States automobiles have on-board EVAP systems that include a canister for collecting and storing fuel vapors evaporating from the engine fuel tank. The canister is purged of these vapors by allowing air to enter the storage canister upstream from a canister outlet to create an air/fuel mixture of unknown air to fuel ratio. The canister air/fuel mixture is drawn into the engine for combustion during engine operation, while the engine is already running under a primary air and fuel control. Accordingly, the air/fuel mixture coming to the engine from the canister supplements the engine""s primary air/fuel feed. In other words, it adds fuel and air to the primary air/fuel feed. However, before purge of the canister starts, the primary air/fuel mixture being fed to the engine is already balanced to a predetermined optimum. Introducing the purge vapors to it will upset this balance.
The volume of air/fuel mixture exiting the canister is substantially fixed. However, the ratio of air to fuel in the mixture is not. To provide more precise fuel control for the engine, it is desirable to know the air to fuel ratio of the mixture as it exits the canister, and then concurrently adjust the primary fuel control to compensate for it. The primary fuel control can then add or reduce its primary fuel supply so that the total resultant air/fuel mixture combusted in the engine is at the desired level even during canister purge. This is a forward-looking method of fuel control, as compared to a reactive fuel control in which fuel is controlled in response to a signal from an exhaust gas oxygen sensor.
I recognize that vapor pressure of the fuel in the mixture exiting the canister is a measure of mixture air/fuel ratio. If fuel vapor pressure in the exiting mixture is measured during purge, a more forward control of engine air/fuel ratio can be obtained. As indicated above, knowing the vapor pressure, i.e., concentration, of the fuel in the air coming from the canister allows one to concurrently adjust the primary air/fuel feed a corresponding amount. One does not have to wait for feedback from the exhaust oxygen sensor to adjust the primary air/fuel feed. Accordingly, this provides a forward control of air/fuel ratio to the engine that offers a variety of benefits.
EVAP systems, also referred to herein as systems, include on-board refueling vapor recovery (ORVR) systems. These systems may include a vapor line from an automobile fuel tank to a canister filled with fuel-adsorbing material, usually carbon. They also include a valved vapor line from the canister to the intake manifold of the internal combustion engine of the automobile. During refueling of the automobile fuel tank, fuel vapors in the fuel tank pass through the fuel tank vapor line to the canister, where they are trapped. The vapor line not only reduces ambient emission of fuel tank vapors from refueling but also during subsequent oration of fuel in the tank. The fuel vapors trapped in the canister are purged from the adsorbing material from time to time during engine operation, for consumption by the engine along with the engine""s primary source of fuel. The fuel, i.e., hydrocarbons, in the purge vapor can become a significant source of fuel fed to the engine. It is, therefore, important to know the hydrocarbon concentration in the purge vapor to obtain better control of fuel introduced into the engine.
In the past, some ORVR systems included complex on-board means for determining fuel vapor pressure. U.S. Pat. No. 5,054,460 Ogita and U.S. Pat. No. 5,111,796 Ogita each describe measuring vapor pressure of automobile fuel by placing a rotary vaned sensor in a vapor line from the automobile fuel tank to an EVAP canister. Fuel vapors passing along the line rotate the sensor, which emits electrical pulses indicating rate of rotation. The Ogita sensor only gives a broad indication of vapor pressure in the tank. It does not indicate what the fuel vapor pressure, especially RVP, is in the purge vapors from the EVAP canister.
In my earlier U.S. Pat. No. 5,884,610 Reddy, I determine RVP of the fuel vapor pressure in the purge vapors rather precisely, using an estimating technique. In my U.S. Pat. No. 5,884,610 Reddy, I describe an engine control system that remembers engine performance under given control conditions immediately before the purge valve on the EVAP canister is opened. That performance is then compared to engine performance immediately after the purge valve is opened. Based on the difference in engine performance, a valuable estimate of fuel vapor pressure is made. Based on this estimate, a signal is fed back to the primary air/fuel ratio control system to appropriate it adjust it for maintaining optimum air/fuel delivery to the engine. However, the system operates after purge starts, by observing its effect on engine performance estimating what is in the purge vapors, and then issuing an estimated adjustment signal to the primary fuel control.
I have now discovered a technique by which fuel vapor pressure can be directly measured, not just estimated. In some instances, such as EVAP systems, the measurement can be done in the EVAP canister itself or in the vapor line from the EVAP canister to the engine. This allows one to not only know fuel vapor pressure in the fuel tank but also to know the fuel vapor pressure, i.e., concentration, of fuel in EVAP canister purge vapors. In other systems, measurement might be in an automobile fuel tank or other fuel storage tank. The measurement is direct, not estimated.
Still further, this invention is also useful for diagnostic identification of fuel in an automobile by the primary fuel delivery system or by a service technician working on a fuel problem. This invention can also be used to quickly identify fuel volatility in other forms of storage and shipping tanks. This can be a significant help to shippers in confirming identity on site of what they think should be in a storage or transfer container. Still further, this invention need not be limited to measurement of fuel vapor pressure. Vapor pressure of various other volatile liquids could be analogously measured as well.
The object of the present invention is to provide an accurate measurement of the vapor pressure of a volatile liquid by galvanically measuring oxygen concentration in a mixture of air and vapors of that liquid. The measurement is made by simply exposing an ordinary oxygen sensor to a mixture of air and vapors of the liquid at atmospheric pressure. The sensor provides an output voltage that is a measure of oxygen partial pressure in the air/vapor mixture. The oxygen to nitrogen ratio in air is a constant, of about 1 to 4. Multiplying the oxygen partial pressure by that constant (i.e., about 5) is an effective measure of air partial pressure in the mixture. Subtracting the air partial pressure from atmospheric pressure provides a measure of fuel vapor partial pressure.
For applications where more accuracy is desired, such as automotive fuel control applications, it may be desirable to concurrently also measure temperature, and perhaps even atmospheric pressure. For many applications, fuel vapor pressure can be adequately characterized by considering that atmospheric pressure is a fixed quantity, such as a fixed reference voltage, representing a standard pressure of 760 mm of mercury or 14.7 pounds per square inch at room temperature. However, if a more precise determination is desired, atmospheric pressure can actually be measured and then used as the comparison quantity. Compensation can be made electrically for deviation in measuring temperature from the reference room temperature. If desired, the fuel vapor pressure measured by my oxygen meter can be converted to reid vapor pressure (RVP) according to the following formula:
RVP=1/(A*T)EXP(ln Pg+2731.41/T)xe2x80x83xe2x80x83(1)
where: A is 21.51
B is 2731.41
Pg is vapor pressure of fuel in pounds per square inch at the measuring temperature T
T is the Pg measuring temperature in degrees Kelvin
Hence, measurement of vapor pressure by my technique is readily usable in various vapor pressure units or forms, as might be desired.